1. Technical Field
The invention relates generally to optical sensors, and particularly to interferometric sensors for determining environmental effects.
2. Description of the Related Art
As fiber optics become more prevalent, various types of optical sensors have become increasingly common. Indeed, various types of sensors detect fiber lengths, locations of breaks/cracks/inconsistencies in optical fibers, temperature, pressure, fiber expansion, attributes of chemical species, and the like.
Optical fibers may be subjected to various external effects that produce geometric (e.g., size, shape) and/or optic (e.g., refractive index, mode conversion) changes to the fiber depending upon the nature and the magnitude of the perturbation. While these effects are often considered to be parasitic (i.e., noise-causing) in communications applications, the fibers response to external influence may be increased in sensing applications so that the resulting change in optical characteristics can be used as a measure of the external effect.
Optical fibers may therefore act as transducers that convert effects such as temperature, stress, strain, rotation or electric and magnetic currents into corresponding changes in optical effects. Since amplitude (intensity), phase, frequency and polarization typically characterize light, any one or more of these parameters may undergo a change due to external effects. The usefulness of the fiber optic sensor therefore depends upon the magnitude of this change and upon the ability to measure and quantify the change reliably and accurately.
Many different types of sensors based upon fiber optic technologies are well known. Among the sensor techniques that have been known for some time are interferometers, which typically detect various phenomena by sensing phase changes or interference patterns between multiple optical signals passing through the sensor. Interferometers have been widely used in the past to determine distance, slope, rotation, and the like. Since about 1980, interferometric fiber optic gyroscopes (IFOGs) have been widely used to detect rotation. Such sensors have proven to be particularly useful for generating inertial navigation data that can be used to guide aircraft, automobiles, other vehicles, downhole drilling apparatus, robots and the like. Various embodiments of IFOGs are described in many patents, including U.S. Pat. Nos. 6,211,963 and 6,175,410, which are incorporated herein by reference.
Fiber optic strain sensors based upon fiber Bragg gratings have similarly been used for several years. Such sensors typically immerse a Bragg grating coupled to an optical sensor into an environment to be sensed. As the environment changes the optical properties of the Bragg grating, the wavelength of light reflected off the grating changes. Hence, an output based upon the wavelength of reflected light may be indicative of some property of the environment sensed. Such sensors have exhibited a number of marked disadvantages, however, in that they are typically expensive, difficult to manufacture, and require measurements of light wavelength, which is difficult to measure accurately in practice. Accordingly, sensors based upon Bragg gratings have typically not been suitable for most low-cost applications. A relatively simple and low-cost interferometric sensor that is accurate and that has a high resolution is therefore desired for a variety of applications.
According to various exemplary embodiments, a technique for sensing an environmental effect upon a sensing element suitably includes exposing the sensing element into the environmental effect, producing a light signal in the sensing element, modulating the light signal with a modulation xe2x80x9cdrivexe2x80x9d signal, and determining a path length of the light signal from the resulting modulated signal on a sensing optical detector, as a function of the modulation drive signal. According to further exemplary embodiments, a fiber optic sensor suitably includes a light source producing a light, a sensing element optically coupled to the light source such that the light propagates through the sensing element, a detector optically coupled to the sensing element. The detector is configured to detect the intensity of the light propagating in the sensing element and to produce a detector output indicative of the intensity; and the electronics receive the detector output and produce a modulation signal for the light based on it, and the electronics further produce an output signal indicative of the environmental effect as a function of the modulation signal.